FIG. 1 shows a circuit diagram of a conventional direct-current converter of this kind (see Harada, Kosuke. Suicchingu Dengen Handobukku (Switching Power supply Handbook), Chapter 2 Suicchingu Dengen no Kihon Kairo to Sekkei Ensyu (Basic Circuits and Design Practice for Switching Power Supply), p 27, FIG. 2.2, Nikkan Kogyo Shinbun Ltd., and Shimizu, Kazuo. Kosoku Suicching Regyureta (High-Speed Switching Regulator), 2.2.1 Tareigata Konbata (Separately Excited Converter), p 30, FIG. 2.5, Sogo Denshi Syuppan). In the direct-current converter shown in FIG. 1, a main switch Q1, including MOSFET or the like, is connected to a direct-current power supply Vdc1 through a primary winding P (winding number: n1) of a transformer T. A parallel circuit including a resistor R2 and a snubber capacitor C2, and a diode D2 connected in series to this parallel circuit are connected to both ends of the primary winding P, respectively. Not only a diode D1 but also a serial circuit including a resistor R1 and a capacitor C1 are connected to both ends of the main switch Q1. The main switch Q1 is turned on/off by PWM control of a control circuit 100.
The primary winding P of the transformer T and a secondary winding S of the same are wound so that common mode voltages are mutually generated. A rectifying/smoothing circuit, including diodes D5 and D6, a reactor L1 and a capacitor C5, is connected to the secondary winding S (winding number: n2) of the transformer T. This rectifying/smoothing circuit rectifies and smoothes a voltage induced by the secondary winding S of the transformer T (an on/off controlled pulse voltage) and produces a direct-current output to a load RL.
The control circuit 100 has a non-illustrated operational amplifier and photocoupler. The operational amplifier compares an output voltage from the load RL to a reference voltage and, where the output voltage from the load RL is the reference voltage or larger, controls so that a turning-on pulse width applied to the main switch Q1 becomes narrower. In other words, where an output voltage from the load RL is the reference voltage or larger, the turning-on pulse width is narrowed, thus controlling output voltages so that they become constant.
Next, operations of the direct-current converter having the above-mentioned construction are described with reference to the timing chart shown in FIG. 2. FIG. 2 shows an operating waveform during light load, depicting a voltage Q1v between both ends of the main switch Q1, a current Q1i flowing through the main switch Q1, and a gate signal Q1g performing the on/off control of the main switch Q1.
First of all, once the main switch Q1 is turned on at time t31, the current Q1i flows through the main switch Q1, performing a circuit in that the current flows through Vdc1, P, Q1, and back through Vdc1. This current Q1i increases linearly until time t32 as time elapses. Further, between time t31 and t32, the main switch Q1 side of the primary winding P becomes the negative side, and, since the primary winding P and the secondary winding S are in the common mode, the anode side of the diode D5 becomes the positive side. Therefore, a current flows through S, D5, L1, C5 and back through S, transmitting energy to the secondary winding side.
Next, once the main switch Q1 is turned off by the gate signal Q1g at time t32, the capacitor C1 is charged with exciting energy induced by the primary winding P of the transformer T and exciting energy of a leakage inductor Lg (an inductance not coupled to the secondary winding S). When a voltage of the capacitor C1 and that of the snubber capacitor C2 are equal, the diode D2 is turned on, and the energy thereof is stored in the snubber capacitor C2. The energy stored in the snubber capacitor C2 is lost by the reactor R2.
Moreover, since a current flowing through the reactor L1 is cut off during light load, once the energy stored in the primary winding P of the transformer T is released completely, the inductance of the primary winding P of the transformer T resonates with the capacitor C1. Hence, the voltage Q1v of the main switch Q1 oscillates as shown in FIG. 2.